The invention applies to any optical instrument comprising a mirror requiring a very great thermal stability, in order to limit the thermoelastic deformations, in particular over short periods, for example one to two hours.
The invention applies most particularly but not exclusively to the optical instruments used in the space field such as the optical instruments on board satellites (typically telescopes).
Specifically, certain optical instruments such as telescopes require a very great geometric stability of their primary mirror at ambient temperature, both over the long term and short term.
With the use of new ceramic materials (silicon carbide-based: CeSiC, SiC, etc.) for producing mirrors, this constraint results, amongst other things, in a high thermal stability in terms of variation of gradient in the thickness of the mirror and in terms of temperature fluctuation of the active face. These mirrors, called primary mirrors, situated in an entrance cavity of the instrument, are subjected directly or indirectly to the external flux variations (solar, terrestrial or albido) on the orbit, and all the year round.
Hitherto, the thermal regulation of such mirrors has been provided by an active regulation of the rear face radiative type. “Active” regulation is carried out conventionally by heaters driven by thermostats or by onboard software coupled to thermistors. This type of regulation makes it possible to maintain the temperature of the mirror at a defined level and to compensate for the variations of flux absorbed by the front face during the year. On the other hand, this type of regulation does not make it possible to compensate for the orbital fluctuations in the context of a satellite in low Earth orbit, because of the purely radiative mode of exchange between the heaters and the mirror.
Other solutions, of the active optical type, exist but are costly and complex to apply because of the use of dedicated electronics and complex functional tests on the ground, and they have a risk of failure.
A direct thermal regulation of the active face would make it possible to obtain an equivalent level of stability of the mirror, but this solution is, to the knowledge of the applicant, not achieved and would present risks of thermoelastic deformation.